Abstract
Structural, magnetic, and computational data on two new Mn(III) complex cations in lattices with five different counterions and varying levels of solvation are compared to investigate the influence of the position of the electron-withdrawing ligand nitro-substituent on the thermal spin crossover profile. The [MnL1]+ (a) and [MnL2]+ (b) complex cations were prepared by complexation of Mn(III) to the Schiff base chelates formed from condensation of 3-nitrosalicylaldehyde or 5-nitrosalicylaldehyde, respectively, with 1,2-bis(3-aminopropylamino)ethane and were crystallized with NO3−, (1a/b), ClO4−, (2a/b), PF6−, (3a/b), CF3SO3− (4a/b), and BPh4− (5a/b) counterions. Magnetostructural analysis reveals a minor trend in the spin state preference depending on the position of the nitro substituent where the orientation is dependent on crystal packing. Compounds using the 3-NO2-sal2-323 ligand, H2L1, where the nitro group is bent out-of-plane to the benzene ring of the Schiff base, tend to stabilize the triplet state, S = 1, while those with the 5-NO2-sal2-323 Schiff base, H2L2, where the nitro group is almost perfectly aligned in-plane with the benzene ring, mostly stabilize the spin-quintet form, S = 2. Density functional theory calculations correctly describe the energetics of intermediate spin/high spin transitions in the complexes. The intrinsic molecular magnetic properties are only marginally dependent on the position of the nitro groups; the out-of-plane orientation for the 3-NO2 is stabilized by an intramolecular hydrogen bonding interaction between the nitro group and the equatorial sal ligand. This demonstrates that the control of magnetic behavior in the solid state is mainly a cooperative effect from the size and distribution of counterions, crystal packing, and intermolecular interactions. Three potential structural phase transitions were identified, in complexes 3a, 4a, and 5a · 2MeCN including one which is not coupled to a spin state change. Finally, a break in the pattern of Jahn-Teller distortion typical for [Mn(R-Sal2-323)]+ complexes was observed in two of the complexes with the 5-NO2 ligand, where elongation of the Mn–O bond lengths on warming replaced the usual pattern of elongation of only Mn–N distances.
Highlights
Two Schiff base ligands with nitro groups appended ortho (H2L1) or para (H2L2) to the phenolate oxygen donor were prepared by addition of 3- or 5-nitrosalicylaldehyde, respectively, in a 2:1 ratio with 1,2-bis(3-aminopropylamino)ethane (Scheme 1)
With the orthosubstituted Schiff base ligand, 3-NO2-sal2-323, the tendency is toward stabilization of the S = 1 spin state in complexes (1a)–(5a), in contrast to stabilization of the S = 2 state with the para-substituted Schiff base analog 5-NO2-sal2-323 in (1b)–(5b)
The difference in spin state preferences could be correlated with the difference in twist angles of the nitro groups, which in turn was related to the ease of forming short contacts with adjacent hydrogen atoms
Summary
Facile access to more than one arrangement of d-electrons in spin crossover (SCO) transition metal complexes constitutes one of the most dramatic and highly developed examples of molecular bistability. SCO complexes are prized because of the ease by which they can be switched by many varied physical stimuli and because the profound differences in spectral and magnetic signatures between the two spin states make the switching easy to follow on a range of timescales. The materials processing of SCO is the subject of intense investigation with many reports of surface attachment and primitive device preparation. Bousseksou et al have recently shown that the challenge of sample fatiguability can be overcome by using high quality vacuum-deposition to prepare a thin film of an Fe2+ SCO complex, which is stable for more than one year and which can endure 107 switching cycles without loss of signal. This development has demonstrated that integration of scitation.org/journal/jap spin switchable complexes into current technologies such as nanothermometry is an achievable goal and will encourage further work toward this important outcome. Despite the impressive advances in moving toward device preparation and real-world applications, less progress has been made in meeting the challenge of predicting or controlling SCO behavior This is true in the solid state where second coordination sphere effects and intermolecular interactions can facilitate or block access to more than one spin state. A systematic analysis of the electronic effects of peripheral ligand substituents on Mn(III) SCO has so far been missing from our work and that with other ligand types.36–38 We address this with a comprehensive magnetostructural comparative study of the effects of positioning strongly electron-withdrawing nitro groups ortho or para to the phenolate oxygen donor in the R-Sal2323 coordination sphere. This results in a strong trend toward stabilization of the spin triplet form for the ortho nitro position in contrast to a tendency for stabilization of the spin quintet form when the nitro group is in the para position
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